FIG. 6 illustrates a conventional thermal printhead (see patent document 1, for example) used as a component of a thermal printer. The illustrated thermal printhead B includes an insulating substrate 91 and a glaze layer 92 made of e.g. glass formed on the substrate. The glaze layer 92 is formed with an electrode 93 and a heating resistor 95. The heating resistor 95 and the electrode 93 are covered by a protection film 96 mainly containing glass material. A platen roller P is provided at a position facing the heating resistor 95.
In printing with the above thermal printhead, the platen roller P presses thermal recording paper S, which is an example of print mediums, onto the protection film 96, while the thermal recording paper S is moved in the secondary scanning direction (right-left direction in FIG. 6). Here, heat generated at the heating resistor 95 is transmitted to the thermal recoding paper S through the protection film 96 for developing color, in other words, printing.
In printing with a thermal printhead, so-called sticking may occur. Sticking is a phenomenon in which the thermal recording paper sticks to the surface of the protection film and thus paper feed of the thermal recording paper is disturbed. The sticking may result in defective printing such as white streaks left on the thermal recording paper.
A method for preventing incidence of sticking may be to reduce friction resistance due to the sliding contact between the thermal recording paper and the protection film. In the conventional thermal printhead shown in FIG. 6, the surface of the protection film 96 is formed to be smooth. Specifically, the protection film 96 has a ten-point mean roughness Rz (JIS B 0601) generally of no more than 0.1 μm. However, even if the surface of the protection film 96 is smooth, the sticking may occur.
For reducing the friction resistance between thermal recording paper and a protection film, a thermal printhead may be arranged in a following manner. Specifically, in this thermal printhead, the protection film has two-layer structure including an insulating layer for covering the heating resistor and the electrode, and a conductive layer for covering the insulating layer (see patent document 2, for example). In this way, static electricity due to contact friction between the surface of the protection film and the thermal recording paper can be efficiently discharged by the conductive layer. Thus, it is possible to prevent the thermal recording paper from adhering to the surface of the protection film due to the static electricity. Still, this arrangement of thermal printhead cannot eliminate the incidence of sticking.
Another method for preventing incidence of sticking may be to reduce the force for pressing the thermal recording paper onto the protection film. However, in this method, heat is not sufficiently transmitted to the thermal recording paper, thereby deteriorating the print quality.
Patent Document 1: JP-A-07-186429
Patent Document 2: JP-A-2001-47652